1. Field of the Invention
The present invention relates to an infrared gas analyzer comprising a pyroelectric infrared sensor, particularly, it relates to an infrared gas analyzer with improved temperature characteristics of the infrared sensor.
2. Description of the Prior Art
An infrared gas analyzer of this type has been disclosed in, for example, "Infrared Gas Analyzer (ULTRAMAT-S)" in Fuji Technical Review Vol. 50, No. 7, Page 368 published in 1977. For an infrared sensor of this type of infrared gas analyzer, conventionally, a microflow type sensor utilizing gaseous heat expansion or a thermistor type sensor utilizing temperature characteristics of the electric resistance has been used.
In the infrared gas analyzer using such an infrared sensor, the change of the ambient temperature does not affect the detecting output of gas, since the same pressure variation occurs in a sample detecting bath and a standard detecting bath for a microflow type sensor. While, in the case of the thermistor type sensor, as the output changes due to temperature variations, the board temperature is retained at a predetermined temperature by an indirectly-heated heater disposed near the board to compensate the temperature characteristics required for the infrared sensor.
In addition to the infrared sensors aforementioned, a pyroelectric infrared sensor may be used. The pyroelectric infrared sensor is designed to detect an infrared ray by irradiating the infrared ray onto a pyroelectric material so as to produce an electric charge by a pyroelectric effect, and thereby measuring the electric voltage produced in the pyroelectric material. In the pyroelectric infrared sensor, if the pyroelectric material temperature itself changes, the electric voltage is produced by a pyroelectric effect even when the infrared ray is not irradiated, so that when using it as the infrared detector, the pyroelectric material temperature and the ambient temperature are preferably kept at a constant as much as possible.
When the pyroelectric infrared sensor is used in the infrared sensor, since it has a higher sensitivity than the microflow type sensor and the thermistor type sensor, a gas analyzer having a higher resolution may be realized, and still the temperature characteristics has to be compensated. Since the pyroelectric infrared sensor is a so-called differential detector in which output is produced by the temperature variations (to the time elapsed) of a pyroelectric element, when arranging the indirectly-heated heater in the vicinity of the pyroelectric element, there is the possibility that sudden temperature variations may cause the sensor to generate an erroneous output, therefore, the sudden change of temperature must be prevented by such a means as increasing the thermal capacity of the indirectly-heated heater.
However, if a material having a high specific heat is wound around the heater to increase the thermal capacity of the indirectly-heated heater, such an indirectly-heated heater becomes larger and can not be contained in the sensor case.
When the infrared ray chopper is a solid-state device and the ultra-small pyroelectric infrared sensor incorporated therein is used (refer to Japanese Patent Application Laid Open No. 82136/1983), it is difficult to install the indirectly-heated heater. Likewise, when the infrared sensor is heated by a heater from outside the sensor case, since heat takes time to reach the infrared detecting unit, a problem is encountered such that the electric heating power of the heater can not be controlled properly even if controlled by detecting the temperature in the sensor case.